Detersive agents such as anionic, cationic, amphoteric and nonionic surfactants are widely used in aqueous based cleansing formulations. In personal care cleansing products (e.g., shampoos, body washes, facial cleansers, liquid hand soaps, hand wipes, etc.), pet care products (e.g., shampoos), household care cleaning products (e.g., hard surface cleaners, laundry detergents, dish soaps, automatic dish washer detergents, shower and bathtub cleansers, bathroom cleansers, car wash detergents, etc.) and industrial and institutional care cleaners (high strength cleaners, detergents, etc.) the surfactant package is the most important component in the detersive formulation. These compositions generally comprise a mixture of one or more surfactants as the active detersive ingredient. The surfactant: 1) improves the wettability of the soiled substrate; 2) loosens soils/oils/sebum from the substrate; and 3) emulsifies, solubilizes and/or suspends the loosened soils/oils/sebum particles in the aqueous wash medium.
Although in principle any surfactant class (e.g., cationic, anionic, nonionic, amphoteric) is suitable as the detersive agent in cleansing or cleaning applications, in practice most personal care cleansers and household cleaning products are formulated with anionic surfactants or with a combination of an anionic surfactant as the primary detersive agent with one or more secondary surfactants selected from the other surfactant classes. Anionic surfactants are used as one of detersive agents in personal care cleansers and detergent cleaning products because of their superior detersive properties. Exemplary anionic surfactants traditionally utilized in these formulations include alkyl sulfates, alkyl benzene sulfonates, and olefin sulfonates. While the anionic surfactants and in particular the anionic sulfates and sulfonates are very efficient detersive agents, they tend to be irritating to the skin and eyes at concentrations typically utilized for efficient detergency. It is widely known that anionic surfactants are adsorbed and even penetrate into the top layers of the skin resulting in irritation to the skin. This irritation is characteristically expressed by reddening of the skin, chapping, scaling, rash development, itching and, in extreme cases, cracking of the skin, or a burning sensation in the eyes.
It has become more and more important to consumers that aqueous cleansing compositions are efficient cleansers as well as mild. These combined properties are especially useful if the cleansing compositions are to be in direct contact with the hair and skin. Consequently, efforts have been made by formulators to deliver personal care cleansing products, household detergents and cleaners and institutional and industrial cleaners that have these properties.
Attempts to impart mildness to cleansers, particularly those formulated for personal care use, involved careful selection of the surfactants employed in the product. It is known that the irritation caused by anionic sulfates can be reduced by introducing ethoxylation into the surfactant molecule. However, a reduction in irritation is accompanied by a corresponding reduction in detergency. For example, sodium lauryl sulfate, a highly detersive surfactant, causes significant eye irritation. In contrast, sodium laureth-12 sulfate (the corresponding ethoxylate containing 12 moles of ethoxylation) is almost completely non-irritating, but is a poor detersive agent (see Schoenberg, “Baby Shampoo,” Household & Personal Products Industry 60 (September 1979)). The poor detersive properties of ethoxylated alkyl sulfates are reported in many other publications.
Additional attempts to attenuate the adverse irritant effects of anionic surfactants have been made by replacing some of the anionic surfactant with very mild secondary surfactants such as amphoteric and/or nonionic surfactants as disclosed in U.S. Pat. No. 4,726,915. However, reducing the amount of anionic surfactant in a cleansing or cleaning composition adversely affects the detersive properties of the composition. The major problem in providing such products resides in the fact that both properties (efficient detergency and mildness) tend to be mutually incompatible. While highly detersive surfactants are generally very harsh, mild surfactants tend to provide insufficient detersive properties.
Another approach for attenuating the adverse irritant effects of anionic detersive surfactants while maintaining high detersive properties in personal care cleansing compositions is disclosed in International Patent Application Pub. No. WO 2005/023970. It is disclosed that certain hydrophobically modified polymeric materials capable of binding surfactant can be combined with anionic surfactants to produce personal care compositions that exhibit relatively low ocular and/or dermal irritation while maintaining high detersive properties. Disclosed hydrophobically modified materials include hydrophobically modified crosslinked acrylic copolymers that are synthesized from at least one ethylenically unsaturated carboxylic acid monomer and at least one ethylenically unsaturated hydrophobically modified monomer. Exemplary hydrophobically modified acrylic polymers are set forth in U.S. Pat. No. 6,433,061 to Noveon, Inc. (now Lubrizol Advanced Materials, Inc.). The disclosure additionally exemplifies polymers available under the trade names Carbopol® Aqua SF-1 and Carbopol® ETD 2020 both provided by Lubrizol Advanced Materials, Inc. as suitable polymers for use as a surfactant binder.
In Pub. No. WO 2005/023970 the applicants therein disclose a relationship between the critical micelle concentration (CMC) of an anionic surfactant in solution and the tendency of the surfactant to induce irritation. The CMC is illustrated by curve 11 in FIG. 1 of the WO 2005/023970 disclosure. As the surfactant is sequentially dosed into a container (of standardized dimension) of water the surfactant initially occupies the surface (liquid/air interface) of the water/surfactant solution. With each sequential dose of surfactant there is a concomitant reduction in the surface tension of the solution until essentially all of the interfacial surface area is filled. Continued dosing of surfactant results in the formation of micelles within the solution. The surfactant concentration at which the further addition of surfactant does not elicit any appreciable affect in solution surface tension is defined as the CMC (point 12 of curve 11). Additional surfactant added after the CMC has been attained was found to induce irritation. In contrast, as illustrated in curve 15 of FIG. 1, as anionic surfactant is added to an aqueous solution comprising a hydrophobically modified crosslinked acrylic polymeric material, the CMC is shifted to a significantly higher surfactant concentration. Accordingly, the inclusion of hydrophobically modified crosslinked acrylic copolymers allows the use of higher concentrations of anionic surfactant in cleansing and cleaning compositions without the attendant ocular and dermal irritation effects.
It is to be noted that the polymers disclosed in U.S. Pat. No. 6,433,061 as well as the polymers identified under the Carbopol® Aqua SF-1 and ETD 2020 trade names are rheology modifiers which thicken or enhance the rheology of the composition in which they are included. In the trade literature Carbopol® Aqua SF-1 polymer is described by Lubrizol Advanced Materials, Inc. Technical Data Sheet TDS-294 (July, 2003) as: “ . . . a lightly crosslinked acrylic polymer dispersion designed to impart suspending, stabilizing, and thickening properties to a variety of surfactant-based personal cleansing products”. The foregoing acrylic based polymers are non-linear (crosslinked), branched polymer chains which interconnect to form three dimensional network structures and have long been used in personal care applications for their rheological and structure building properties. Upon neutralization, these water soluble or dispersible polymers possess the unique ability to greatly increase the viscosity of the liquid in which they are dissolved or dispersed.
The disclosed hydrophobically modified crosslinked acrylic copolymers are viscosity building agents that increase the viscosity of compositions in which they are dissolved or dispersed upon suitable neutralization of the carboxylic acid moieties on the polymer backbone with an alkaline material. As increasing amounts of viscosity builder are added to a cleansing or cleaning formulation to mitigate the adverse irritation effects of the anionic surfactant there is a corresponding increase in the viscosity of the composition. It is well known in the personal care, household care and industrial and institutional care formulation art that a liquid cleanser or cleaner should have an ideal viscosity. Indeed, viscosity allows for the controlled handling and dispensing of the product during use as compared to a thinner product. In personal care cleansing applications, a thick, rich shampoo or body cleanser is appealing to consumers from a sensory perspective. In household care applications, viscosity permits a better efficacy of the product when applied to non-horizontal surfaces such as toilet bowls, sinks, shower stalls, bath tubs, and the like. In addition, cleansing and cleaning products are expected to be easy to use. In other words, the shear thinning profile of the liquid composition should exhibit high viscosity at low shear conditions and lower viscosity at high shear conditions to aid in the application and removal of the product from the substrate to be cleaned.
The commercially available Carbopol® SF-1 polymer and similar polymers comprise a chemically crosslinked backbone having a pH-responsive functionality that is either base or acid sensitive. The polymers may be mixed with other ingredients in a formulation and then neutralized by the addition of a neutralization agent such as an acid or a base. Acid sensitive thickeners are activated upon contact with an acidic agent, while base-sensitive thickeners are activated upon contact with an alkaline agent. Upon neutralization, the polymers swell significantly to form a randomly close-packed (RCP) jammed network of swollen cross-linked micro-gel particles imparting a desired rheological profile, i.e., yield stress, elastic modulus, and viscosity, as well as optical clarity to the formulation.
There are drawbacks associated with increasing the viscosity of a product beyond its ideal viscosity. Highly viscous products are typically difficult to apply and rinse away, especially if the shear thinning profile of the viscosity building agent is poor. High viscosities can also adversely affect packaging, dispensing, dissolution, and the foaming and sensory properties of the product.
While a certain rheology modifier may thicken or enhance the viscosity of a composition in which it is included, it does not necessarily have desirable yield stress properties. A desirable yield stress property is critical to achieving certain physical and aesthetic characteristics in a liquid medium, such as the indefinite suspension of particles, insoluble liquid droplets, or the stabilization of gas bubbles within a liquid medium. Particles dispersed in a liquid medium will remain suspended if the yield stress (yield value) of the medium is sufficient to overcome the effect of gravity or buoyancy on those particles. Insoluble liquid droplets can be prevented from rising and coalescing and gas bubbles can be suspended and uniformly distributed in a liquid medium using yield value as a formulating tool. A yield stress fluid is used generally to adjust or modify the rheological properties of aqueous compositions. Such properties include, without limitation, viscosity improvement, flow rate improvement, stability to viscosity change over time, and the ability to suspend particles for indefinite periods of time.
To alleviate the substantial viscosity profile of the above described crosslinked hydrophobically modified acrylic polymers, U.S. Pat. No. 8,293,845 describes the use of low molecular weight hydrophobically modified linear (non-crosslinked) acrylic polymers to increase the CMC of a surfactant containing composition to mitigate irritation. The applicants teach that the ideal CMC is achieved by neutralizing the linear polymer with an alkaline material to a degree of neutralization ranging from about 15 to about 30%, based on the acid number of the polymer. As with the crosslinked hydrophobically modified acrylic polymer irritation mitigants, these linear acrylic polymer counterparts are pH dependent in that the degree of neutralization of the polymer must be maintained within a narrow range in order to reach the optimal CMC value. In addition, the disclosed linear polymers are not crosslinked and do not create a yield stress.
Accordingly, there is a need for an irritation mitigation polymer that is not pH dependent and that can be tailored to create a desired yield stress in the detersive composition in which it is incorporated.